Display device and electronic device including the same

ABSTRACT

One embodiment of the present invention provides a highly reliably display device in which a high mobility is achieved in an oxide semiconductor. A first oxide component is formed over a base component. Crystal growth proceeds from a surface toward an inside of the first oxide component by a first heat treatment, so that a first oxide crystal component is formed in contact with at least part of the base component. A second oxide component is formed over the first oxide crystal component. Crystal growth is performed by a second heat treatment using the first oxide crystal component as a seed, so that a second oxide crystal component is formed. Thus, a stacked oxide material is formed. A transistor with a high mobility is formed using the stacked oxide material and a driver circuit is formed using the transistor.

TECHNICAL FIELD

The present invention relates to a display device including an oxidesemiconductor, a manufacturing method thereof, and an electronicappliance including the display device.

BACKGROUND ART

Transistors formed over a glass substrate or the like are typicallymanufactured using amorphous silicon, polycrystalline silicon, or thelike, as typically seen in liquid crystal display devices. Althoughtransistors including amorphous silicon have low field effect mobility,they can be formed over a larger glass substrate. On the other hand,although transistors including polycrystalline silicon have high fieldeffect mobility, they are not suitable for being formed over a largerglass substrate.

In view of the foregoing, attention has been drawn to a technique bywhich a transistor is manufactured using an oxide semiconductor, andsuch a transistor is applied to an electronic appliance or an opticaldevice. For example, a technique of manufacturing a transistor by usingzinc oxide or In—Ga—Zn—O-based oxide as oxide semiconductor, and usingthe transistor for a switching element of a pixel of a display deviceand the like is disclosed in Patent Document 1 and Patent Document 2.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2007-123861-   [Patent Document 2] Japanese Published Patent Application No.    2007-096055

DISCLOSURE OF INVENTION

The field effect mobility of a transistor in which a channel region isformed in an oxide semiconductor is higher than that of a transistor inwhich a channel region is formed in amorphous silicon. The field effectmobility of a transistor formed using amorphous silicon is approximately0.5 cm²/Vs in general, whereas the field effect mobility of a transistorformed using an oxide semiconductor is 10 cm²/Vs to 20 cm²/Vs. Inaddition, an active layer can be formed using an oxide semiconductor bya sputtering method or the like, and a manufacturing process thereof issimpler than that of a transistor including polycrystalline siliconformed using a laser device.

Transistors which are manufactured using such an oxide semiconductorover a glass substrate or a plastic substrate are expected to be appliedto liquid crystal display devices, organic EL display devices,electronic paper, and the like.

Meanwhile, large-sized display devices are spreading. Televisions withdisplay screens having a diagonal of 40 inches to 50 inches are becomingcommon as home televisions and are expected to become more popular inthe future. As described above, a transistor formed using an oxidesemiconductor has a field effect mobility which is 10 or more times ashigh as that of a transistor formed using amorphous silicon; therefore,the transistor formed using an oxide semiconductor can have propertiessufficient to be used as a switching element of a pixel even in alarge-sized display device.

However, when not only a pixel but also a driver circuit is formed usinga transistor formed using an oxide semiconductor, a transistor formedusing a conventional oxide semiconductor does not have sufficientcharacteristics. Specifically, the field effect mobility of theconventional oxide semiconductor needs to be increased by several timesin order to improve the current capability of a transistor. In the casewhere a driver is formed using a transistor formed using an oxidesemiconductor with a field effect mobility of 10 cm²/Vs, the size of adisplay device should be smaller than 20 inches; therefore, when alarger-sized display device is manufactured, a driver needs to beadditionally mounted.

An embodiment of the present invention disclosed in this specificationis an active matrix display device including a plurality of pixels, aplurality of signal lines, and a plurality of scan lines over aninsulating substrate. The display device includes a transistor formedusing an oxide semiconductor with a field effect mobility of at least 50cm²/Vs or higher, preferably 100 cm²/Vs or higher over the insulatingsubstrate. The display device also includes a gate driver and an analogswitch for driving a source line each including the transistor as one ofits elements.

The size of the above display device is at least 20 inches.

An embodiment of the present invention disclosed in this specificationis an active matrix display device including a plurality of pixels, aplurality of signal lines, and a plurality of scan lines over aninsulating substrate. The display device includes a transistor formedusing an oxide semiconductor with a field effect mobility of at least 50cm²/Vs or higher, preferably 100 cm²/Vs or higher over the insulatingsubstrate. The display device also includes a gate driver and a sourcedriver each including the transistor as one of its elements.

The size of the above display device is at least 20 inches.

One method for improving a field effect mobility is a method formanufacturing a stacked oxide material, including the steps of forming alayer of an oxide component over a base component, performing crystalgrowth which proceeds from a surface toward an inside of the oxidecomponent by heat treatment to form a first oxide crystal component atleast partly in contact with the base component, and stacking a secondoxide crystal component over the first oxide crystal component. Inparticular, the first oxide crystal component and the second oxidecrystal component have common c-axes. Note that the first oxide crystalcomponent has its c-axis-aligned perpendicularly to a surface of thefirst oxide crystal component. Note that on an a-b plane, elementsadjacent to each other are identical. In addition, the c-axis directionof the first oxide crystal component corresponds to the depth direction.

In the above manufacturing method, at least part or the whole of abottom surface of the first oxide crystal component in which crystalsare aligned is provided to be in contact with the base component. Byappropriately adjusting the thickness of the oxide component, heatingconditions, or the like, the bottom surface of the first oxide crystalcomponent in which crystals are aligned is provided so that at leastpart or the whole of the bottom surface is in contact with the basecomponent.

In the above manufacturing method, annealing is performed afterdeposition of the first oxide component as a first oxide semiconductorlayer and a second oxide component is deposited as a second oxidesemiconductor layer over a top surface of the first oxide crystalcomponent. After that, a crystal is grown from the interface between thefirst oxide semiconductor layer and the second oxide semiconductor layertoward a surface of the second oxide semiconductor layer which is anupper layer. The first oxide crystal component corresponds to a seedcrystal for the second oxide component. It is important to form, abovethe first oxide crystal component, the second oxide crystal component asa polycrystalline layer.

As crystallinity of an oxide semiconductor layer is higher, a transistorwith high field effect mobility can be realized.

As the crystallinity of the oxide semiconductor layers is increased, theamount of change in the threshold voltage of a transistor before andafter a BT test can be reduced, so that high reliability can beobtained.

In addition, as the crystallinity of the oxide semiconductor layers isincreased, temperature dependence of electric characteristics of thetransistor, for example, the amount of change in on current or offcurrent at temperatures of from −30° C. to 120° C. can be reduced.

One feature of the above structure is that the oxide crystal componentin which a c-axis-aligned crystal is in contact with the base componentis a polycrystalline component.

A technical idea of the present invention is that an impurity is notadded to an oxide semiconductor and on the contrary, the oxidesemiconductor itself is highly purified by intentionally removing animpurity such as water or hydrogen which undesirably exists therein. Inother words, the technical idea is to highly purify an oxidesemiconductor by removing water or hydrogen which forms a donor level,reducing oxygen deficiency, and sufficiently supplying oxygen that is amain component of the oxide semiconductor.

When an oxide semiconductor is deposited, hydrogen at density of 10²⁰cm⁻³ is measured using secondary ion mass spectrometry (SIMS). The oxidesemiconductor is highly purified and made to be an i-type (intrinsic)semiconductor by intentionally removing water or hydrogen which forms adonor level and simultaneously adding oxygen (one of the components ofthe oxide semiconductor).

Further, in a technical idea of the present invention, it is preferablethat the amount of water and hydrogen in an oxide semiconductor be assmall as possible, and it is also preferable that the number of carriersin the oxide semiconductor be also as small as possible. In other words,a carrier density of less than 1×10¹² cm³, preferably less than1.45×10¹⁰ cm⁻³ which is less than or equal to the measurement limit isneeded. In addition, an ideal career density is 0 or approximately 0 inthe technical idea of the present invention. In particular, when anoxide semiconductor is subjected to heat treatment in an oxygenatmosphere, a nitrogen atmosphere, or an atmosphere of ultra-dry air(air in which the water content is less than or equal to 20 ppm,preferably less than or equal to 1 ppm, more preferably less than orequal to 10 ppb) at a temperature higher than or equal to 450° C. andlower than or equal to 850° C., preferably higher than or equal to 550°C. and lower than or equal to 750° C., water or hydrogen which wouldform an n-type impurity can be removed and the oxide semiconductor canbe highly purified. In addition, when the oxide semiconductor is highlypurified by removing an impurity such as water or hydrogen, the carrierdensity thereof can be less than 1×10¹² cm⁻³, preferably less than1.45×10¹⁰ cm⁻³ which is less than or equal to the measurement limit.

In addition, when the heat treatment is performed at a high temperature,that is, a temperature higher than or equal to 450° C. and lower than orequal to 850° C., preferably higher than or equal to 600° C. and lowerthan or equal to 700° C., the oxide semiconductor can be highly purifiedand also crystallized, and crystal growth proceeds from a surface of theoxide semiconductor toward the inside thereof, so that the oxidesemiconductor has polycrystalline regions whose c-axis is aligned.

As for the oxide semiconductor used in the present invention, the oxidesemiconductor having polycrystalline regions whose c-axis is aligned isused as a seed crystal, a second oxide semiconductor is formedthereover, and heat treatment is performed at a temperature higher thanor equal to 450° C. and lower than or equal to 850° C., preferablyhigher than or equal to 550° C. and lower than or equal to 750° C., sothat the second oxide semiconductor can have a polycrystalline regionwhose c-axis is aligned in a manner similar to that of the seed crystal.That is to say, ideal axial growth or epitaxial growth in which the seedcrystal and the second oxide semiconductor have c-axes which are alignedin the same direction can be performed.

The second oxide semiconductor having the same axis as the seed crystalcan be stacked and also the crystal thereof can be grown not only byperforming solid-phase growth by heat treatment after deposition butalso by depositing with the use of typically sputtering in a state whereheating is performed at a temperature higher than or equal to 200° C.and lower than or equal to 600° C. Further, when a substrate is heatedat higher than or equal to 200° C. and lower than or equal to 600° C. information of the oxide semiconductor film by a sputtering method,epitaxial growth or axial growth can be performed.

In addition, carriers in the oxide semiconductor are reduced, orpreferably all carriers are removed, so that the oxide semiconductorserves as a path through which carriers pass in a transistor. As aresult, the oxide semiconductor is a highly purified i-type (intrinsic)semiconductor which has no carrier or very few carriers, so that offcurrent of a transistor in an off state can be extremely low, which isthe technical idea of the present invention.

In addition, when the oxide semiconductor serves as a path and the oxidesemiconductor itself is a highly purified i-type (intrinsic)semiconductor which has no carriers or very few carriers, carriers aresupplied by a source electrode and a drain electrode. When the electronaffinity χ, the Fermi level, preferably corresponding to the intrinsicFermi level in the oxide semiconductor, and the work functions of thesource and drain electrodes are selected as appropriate, carriers can beinjected from the source electrode and the drain electrode. Therefore,an n-channel transistor and a p-channel transistor can be manufacturedas appropriate.

All of the oxide crystal components and the oxide components are formedusing metal oxides, and any of the following metal oxide films can beused: a four-component metal oxide film such as an In—Sn—Ga—Zn—O-basedfilm; a three-component metal oxide film such as an In—Ga—Zn—O-basedfilm, an In—Sn—Zn—O-based film, an In—Al—Zn—O-based film, aSn—Ga—Zn—O-based film, an Al—Ga—Zn—O-based film, or a Sn—Al—Zn—O-basedfilm; a two-components metal oxide film such as an In—Zn—O-based film, aSn—Zn—O-based film, an Al—Zn—O-based film, a Zn—Mg—O-based film, aSn—Mg—O-based film, or an In—Mg—O-based film; or an In—O-based film, aSn—O-based film, or a Zn—O-based film. Note that here, for example, anIn—Sn—Ga—Zn—O-based film means a metal oxide film including indium (In),tin (Sn), gallium (Ga), and zinc (Zn) and there is no particularlimitation on the stoichiometric proportion.

As the oxide crystal components and the oxide components, a thin filmexpressed by InMO₃(ZnO)_(m) (m>0, and m is not a natural number) can beused. Here, M represents one or more metal elements selected from Ga,Al, Mn, and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga andCo, or the like.

In addition, an oxide semiconductor material expressed by In-A-B—O maybe used. Here, A represents one or plural kinds of elements selectedfrom elements belonging to Group 13, such as gallium (Ga) or aluminum(Al), elements belonging to Group 14 typified by silicon (Si) orgermanium (Ge), and the like. In addition, B represents one or pluralkinds of elements selected from elements belonging to Group 12 typifiedby zinc (Zn). Note that the In content, the A content, and the B contentare set freely, and the case where the A content is 0 is included. Onthe other hand, the In content and the B content are not 0. In otherwords, the above expression includes In—Ga—Zn—O, In—Zn—O, and the like.In addition, an oxide semiconductor material expressed by In—Ga—Zn—O inthis specification is InGaO₃(ZnO)_(m) (m>0, and m is not a naturalnumber), and it can be confirmed using analysis with ICP-MS or RBS thatm is not a natural number.

In addition, as one step for high purification, first heat treatment isperformed in an atmosphere which hardly contains hydrogen and moisture(such as a nitrogen atmosphere, an oxygen atmosphere, or a dry-airatmosphere (for example, as for the moisture, a dew point is lower thanor equal to −40° C., preferably lower than or equal to −50° C.)). Thefirst heat treatment can be called dehydration or dehydrogenation, whichis for removing H, OH, or the like from the oxide semiconductor layer.In the case where temperature is raised in an inert atmosphere and thenthe atmosphere is switched to an atmosphere containing oxygen duringheat treatment, or in the case where an oxygen atmosphere is employed,the first heat treatment can also be called additional oxidationtreatment.

For the first heat treatment for dehydration or dehydrogenation, aheating method using an electric furnace, a rapid heating method such asa gas rapid thermal anneal (GRTA) method using a heated gas or a lamprapid thermal anneal (LRTA) method using lamp light, or the like can beused. In addition, as the first heat treatment, heating with irradiationwith light having a wavelength less than or equal to 450 nm may beperformed at the same time. The oxide semiconductor layer which issubjected to the first heat treatment for high purification is heatedunder conditions that at least a peak at around 300° C. of two peaks ofwater are/is not detected when the oxide semiconductor layer after thefirst heat treatment is measured with thermal desorption spectroscopy(TDS) while the temperature is increased to 450° C. Therefore, even ifTDS is performed at up to 450° C. on a transistor including the oxidesemiconductor layer which is subjected to heat treatment for highpurification, at least the peak of water at around 300° C. is notdetected.

Since crystal growth is performed in a state where there is nopolycrystalline layer which serves as a seed of crystal growth, it ispreferable that the first heat treatment be performed at hightemperature for a short time so that only crystal growth from a surfaceis performed. In addition, when a surface of the oxide semiconductorlayer is flat, a favorable plate-shaped polycrystalline layer can beobtained. Therefore, it is preferable that flatness of a base componentsuch as an insulating layer or a substrate be as high as possible.Increase of flatness is effective because a polycrystalline layer incontact with an entire surface of the base component can be formedeasily. For example, the flatness of the oxide semiconductor layer isapproximately equivalent to that of a commercial silicon wafer; forexample, difference in height of surface roughness in a region of 1 μm×1μm by AFM measurement is less than or equal to 1 nm, preferably 0.2 nm.

In the polycrystalline layer, when electron clouds of In included in theoxide semiconductor overlap with each other and are connected to eachother, electric conductivity σ is increased. Accordingly, a transistorincluding the polycrystalline layer can have high field effect mobility.

One of methods for further performing crystal growth with the use of theplate-shaped polycrystalline layer formed by the first heat treatment asa seed is described below with reference to FIGS. 1A, 1B, and 1C.

The outline of the order of steps is as follows: the first oxidesemiconductor layer is formed over the base component; first heattreatment for high purification is performed; the polycrystalline layerwhose crystal direction is aligned is formed over a surface of the firstoxide semiconductor layer in the same step as the first heat treatmentfor high purification; the second oxide semiconductor layer is stackedthereover; and further, second heat treatment for crystallization isperformed, so that the second oxide semiconductor layer is crystallizedwith the use of the polycrystalline layer over the surface of the firstoxide semiconductor layer as a seed.

In the first heat treatment, crystal growth is performed from thesurface in the state where a crystal layer serving as a seed of crystalgrowth is not present, whereas in the second heat treatment, theplate-shaped polycrystalline layer serving as a seed is present.Therefore, it is preferable that the second heat treatment be performedfor a long time at a lowest temperature at which crystal growth can beperformed because favorable crystallinity can be obtained. Thecrystal-growth direction obtained by the second heat treatmentcorresponds to an upward direction from a lower portion, i.e., adirection from the substrate side to the surface side (also referred toas recrystallization direction) and is different from that in the firstheat treatment. In addition, since the polycrystalline layer obtained bythe first heat treatment is heated again by the second heat treatment,the crystallinity of the polycrystalline layer is further increased.

FIG. 1A illustrates a state where first heat treatment forcrystallization is performed on a first oxide semiconductor layer formedover a base component 500. The first heat treatment is performed in anoxygen atmosphere, a nitrogen atmosphere, or an ultra-dry air atmosphereat higher than or equal to 450° C. and lower than or equal to 850° C.,preferably higher than or equal to 550° C. and lower than or equal to750° C. Further, heat treatment may be performed such that temperatureis raised in an inert gas atmosphere and then the atmosphere is switchedto an atmosphere containing oxygen, or heat treatment in an oxygenatmosphere may be performed. After the first heat treatment, the firstoxide semiconductor layer becomes a first oxide crystal component 501 ofa plate-shaped polycrystal which is c-axis-aligned perpendicularly to asurface.

FIG. 1B is a cross-sectional view of a state just after formation of asecond oxide semiconductor layer 502. The second oxide semiconductorlayer 502 is formed by a sputtering method, and a metal oxide targetsuch as a metal oxide target with In:Ga:Zn=1:1:1 [atomic ratio] or ametal oxide target with In:Ga:Zn=1:1:2 may be used.

FIG. 1C illustrates a cross-sectional view posterior to the second heattreatment. By the second heat treatment, crystal growth proceeds upwardtoward a surface of the second oxide semiconductor layer 502 with theuse of the polycrystalline layer of the first oxide crystal component501 as a seed. As a result, a second oxide crystal component 503 b isformed, so that all the crystal components are c-axis-aligned.

The second heat treatment can also be called dehydration ordehydrogenation which is for removing H, OH, or the like from the oxidesemiconductor layer. In the case where temperature is raised in an inertatmosphere and then the atmosphere is switched to an atmospherecontaining oxygen, or in the case where an oxygen atmosphere isemployed, the second heat treatment can also be called additionaloxidation treatment.

Further, the polycrystalline layer obtained in the first heat treatmentis heated again in the second heat treatment; accordingly, a third oxidecrystal component 503 a in which crystallinity is further improved isobtained.

The hydrogen concentration in the oxide semiconductor layer is lowerthan or equal to 1×10¹⁸ cm⁻³, preferably lower than or equal to 1×10¹⁶cm⁻³, more preferably substantially 0. The carrier density of the oxidesemiconductor layer is lower than 1×10¹² cm⁻³, preferably lower than1.45×10¹⁰ cm⁻³ which is lower than or equal to the measurement limit.That is, the carrier density of the oxide semiconductor layer is asclose to zero as possible. Furthermore, the band gap of the oxidesemiconductor is greater than or equal to 2 eV, preferably greater thanor equal to 2.5 eV, more preferably greater than or equal to 3 eV. Notethat the hydrogen concentration of an oxide semiconductor layer can bemeasured by secondary ion mass spectrometry (SIMS). The carrier densitycan be measured by the Hall effect measurement. Lower carrier densityN_(d) can be obtained with the use of a measurement result ofcapacitance-voltage (CV) measurement and Formula 1.

$\begin{matrix}{N_{d} = {{- \left( \frac{2}{e\; ɛ_{0}ɛ} \right)}/\frac{\left( {1/C} \right)^{2}}{V}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

In addition, it can be said that FIG. 1C illustrates a two-layerstructure in which the third oxide crystal component 503 a is providedon and in contact with the base component 500 and the second oxidecrystal component 503 b is stacked thereon. There are no particularlimitations on materials of the first oxide crystal component 501 andthe second oxide crystal component 503 b as long as a polycrystal whichis c-axis-aligned perpendicularly to the surface can be obtained. Thematerials of the first oxide crystal component 501 and the second oxidecrystal component 503 b may be different or may contain the samecomponents. “Containing the same components” means that the same elementis included therein.

Note that in the case where oxide semiconductor materials containing thesame components are used, a boundary between the third oxide crystalcomponent 503 a and the second oxide crystal component 503 b is unclearas illustrated by a dotted line in FIG. 1C, so that a single-layerstructure is obtained.

In this manner, the polycrystalline layer formed of a stack of the thirdoxide crystal component 503 a and the second oxide crystal component 503b can be obtained by crystal growth by separately performing two heattreatments.

Note that in FIG. 1A, crystal growth of the plate-shaped crystal layerwhose crystal direction is relatively aligned over the surface of thefirst oxide semiconductor layer proceeds in the depth direction from thesurface; therefore, the polycrystalline layer can be formed withoutbeing influenced by the base component.

An example of a mechanism in which the crystal layer whose crystaldirection is relatively aligned is formed on the surface of the firstoxide semiconductor layer, for example, of an In—Ga—Zn—O film isdescribed. By heat treatment, zinc included in the In—Ga—Zn—O film isdispersed and concentrated at the vicinity of the surface, and becomes aseed of crystal growth. At the time of the crystal growth, crystalgrowth in the horizontal direction (a direction parallel to the surface)more strongly proceeds than crystal growth in the depth direction (adirection vertical to the surface); therefore, the plate-shapedpolycrystalline layer is formed. That is, the first oxide semiconductorlayer is more likely crystallized in the direction of the a-b plane thanin the direction of the c-axis. In addition, a-b planes in crystals donot correspond to each other. In addition, since a space above thesurface of the In—Ga—Zn—O film is a free space, crystal growthproceeding upward in this free space does not occur. These are supposedfrom the fact that when TDS measurement is performed to 450° C., peaksof In and Ga are not detected but a peak of zinc is detected in avacuum-heat condition, in particular at around 300° C. Note that it canbe confirmed that the TDS measurement is performed in vacuum andemission of zinc is detected from a temperature around 200° C.

It can be said that a plate-shaped layer having a large thickness can beformed in such a manner that two depositions of oxide semiconductorfilms are performed so that a polycrystalline layer to be a seed ofcrystal growth is formed and then a second film is deposited, and thencrystal growth is performed. Accordingly, the method disclosed in thisspecification is extremely effective.

In addition, the method is also useful in that a crystal layer which hasthe a-b plane parallel to a surface and is c-axis-aligned in a directionperpendicular to the surface can be obtained regardless of a material ofa base component.

A device formed using a metal oxide, typically an In—Ga—Zn—O film istotally different from a device formed using a single crystal Si, adevice formed using SiC, and a device formed using GaN.

As a wide-gap semiconductor, SiC (3.26 eV) and GaN (3.39 eV) are known.However, SiC and GaN are expensive materials. Further, SiC and GaNrequire a process temperature of 1500° C. or higher; thus, it ispractically impossible to form a thin film thereof over a glasssubstrate.

Further, the crystal structure of SiC or GaN is only a single crystal.Therefore, control with a PN junction is needed and more completesingle-crystallization is needed. Accordingly, since a small amount ofimpurities which are unintentionally mixed in a manufacturing processserves as donors or acceptors, the carrier concentration has lowerlimit. In contrast, a metal oxide can have any of an amorphousstructure, a polycrystalline structure, and a single crystallinestructure. One feature of a metal oxide is that control of a band whichis equivalent to a PN junction is performed by utilizing each ofproperties of φ_(MS) versus χ_(OS)+½Eg^(OS), φ_(MD) versusχ_(OS)+½Eg^(OS), work functions of a source and a drain, electronaffinity of metal oxide, and energy band width, without using controlwith a PN junction.

A metal oxide, typically an In—Ga—Zn—O film has a band gap which isthree times as wide as that of single crystal silicon and is aninexpensive material because of a low manufacturing cost as comparedwith SiC.

The band gap of In—Ga—Zn—O is 3.05 eV. The intrinsic carrier density iscalculated based on this value. It is known that the energy distributionf(E) of electrons in a solid is based on the Fermi-Dirac statisticsrepresented by the following formula.

$\begin{matrix}{{f(E)} = \frac{1}{1 + {\exp \left( \frac{E - E_{F}}{kT} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

In the case of a normal semiconductor whose carrier density is not veryhigh (which does not degenerate), the following relational expression issatisfied.

|E−E _(F) |>kT   [Formula 3]

Accordingly, the Fermi-Dirac distribution of the formula (1) can beapproximated by the formula of the Boltzmann distribution represented bythe following formula.

$\begin{matrix}{{f(E)} = {\exp \left\lbrack {- \frac{E - E_{F}}{kT}} \right\rbrack}} & \left\lbrack {{Formula}\mspace{14mu} 4} \right\rbrack\end{matrix}$

When the intrinsic carrier density (n_(i)) of the semiconductor iscalculated with the formula (3), the following formula can be obtained.

$\begin{matrix}{n_{i} = {\sqrt{N_{C}N_{V}}{\exp \left( {- \frac{E_{g}}{2{kT}}} \right)}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

Then, the values of effective density of states (Nc and Nv) of Si andIn—Ga—Zn—O and the band gaps (Eg) thereof are substituted into theexpression (4) and the intrinsic carrier density is calculated. Theresults are shown in Table 1.

TABLE 1 Si IGZO Nc (300K) [cm⁻³]  2.8 × 10¹⁹ 5.0 × 10¹⁸ Nv (300K) [cm⁻³]1.04 × 10¹⁹ 5.0 × 10¹⁸ Eg (300K) [eV] 1.08 3.05 n_(i) (300K) [cm⁻³] 1.45× 10¹⁰ 1.2 × 10⁻⁷

It is found that In—Ga—Zn—O has extremely low intrinsic carrier densitywhen compared to Si. In the case where the value of 3.05 eV is selectedas a band gap of IGZO, it can be said that the carrier density of Si isapproximately 10¹⁷ times as large as that of In—Ga—Zn—O, assuming thatthe Fermi-Dirac distribution law is applicable to the intrinsic carrierdensity.

In the case of an oxide semiconductor, a thin oxide semiconductor filmcan be formed by a sputtering method at a heating temperature from aroom temperature to 400° C. and a maximum process temperature can be setto a temperature higher than or equal to 300° C. and lower than or equalto 800° C. In the case where the maximum process temperature is set to atemperature lower than or equal to the strain point of glass, a thinoxide semiconductor film can be formed over a large-area glasssubstrate. Therefore, from the standpoint of industrialization, it isimportant to manufacture a metal oxide whose band gas is wide, employinga maximum process temperature higher than or equal to 300° C. and lowerthan or equal to 800° C.

Metal oxides which have been reported so far have an amorphous state, apolycrystalline state, or a single-crystalline state and are obtainedthrough treatment at a high temperature of approximately 1500° C.However, as described above, a thin polycrystal which is c-axis-alignedcan be formed at a relatively low temperature with a method in which aplate-shaped polycrystal of a metal oxide is formed and then crystalgrowth is performed using the plate-shaped polycrystal of the metaloxide as a seed. Further, if formation of a thicker polycrystalline filmis possible, industrial application is further expanded. Note that inorder to obtain a fine thick polycrystalline film, flatness andsmoothness of the substrate are preferably high. This is because smallunevenness of the substrate leads to local shift of the c-axis, whichresults in defects such as crystal transition due to a difference in thec-axis direction between adjacent crystals as crystal growth proceeds.

Note that with the use of the oxide semiconductor layer including theplate-shaped crystal layer, a transistor with high field effect mobilitycan be obtained. In addition, a transistor with low off current can beobtained. In addition, a so-called normally-off switching element can beobtained, so that a semiconductor device with low power consumption canbe provided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C are cross-sectional views illustrating an embodiment ofthe present invention;

FIGS. 2A to 2E are cross-sectional views illustrating manufacturingsteps of an embodiment of the present invention;

FIGS. 3A and 3B are a top view and a cross-sectional view of anembodiment of the present invention;

FIGS. 4A and 4B are views illustrating a display device of an embodimentof the present invention;

FIGS. 5A and 5B are views showing timing of a display device of anembodiment of the present invention;

FIGS. 6A and 6B are views each illustrating a display device of anembodiment of the present invention;

FIG. 7 shows a relation between rise time of a gate line and a size of adisplay device;

FIG. 8 shows writing of a source line;

FIG. 9 is a view illustrating an embodiment of a shift register;

FIGS. 10A and 10B show timing of a shift register;

FIG. 11 shows writing of a source line;

FIG. 12 shows writing of a source line;

FIG. 13 is a longitudinal cross-sectional view of a bottom gatetransistor including an oxide semiconductor;

FIG. 14 is an energy band diagram (schematic diagrams) of a crosssection along line A-A′ illustrated in FIG. 13;

FIG. 15A shows a state in which a positive potential (+V_(G)) is appliedto a gate (GE), and FIG. 15B shows a state in which a negative potential(−V_(G)) is applied to the gate (GE);

FIG. 16 is a view illustrating a relation between the vacuum level and awork function (φM) of a metal and a relation between the vacuum leveland the electron affinity (χ) of an oxide semiconductor;

FIGS. 17A to 17C are cross-sectional views illustrating manufacturingsteps of an embodiment of the present invention;

FIGS. 18A and 18B are a top view and a cross-sectional view illustratingan embodiment of the present invention;

FIGS. 19A and 19B are a top view and a cross-sectional view illustratingan embodiment of the present invention;

FIG. 20 is a cross-sectional view of an embodiment of the presentinvention;

FIGS. 21A to 21E are each an example of an electronic device; and

FIG. 22 is an example of an electronic device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with references to the accompanying drawings. However, thepresent invention is not limited to the description below, and it iseasily understood by those skilled in the art that modes and detailsdisclosed herein can be modified in various ways. Therefore, the presentinvention is not construed as being limited to description of theembodiments.

Embodiment 1

FIGS. 4A and 4B show an embodiment of the present invention. FIG. 4Aillustrates a display device provided with a pixel portion 1502, gatedrivers 1503 and 1504, and an analog switch 1505 for driving a sourceline integrated on a glass substrate 1501. The reason why the analogswitch 1505 is used for driving the source line is as follows. Forexample, in the case of a full high-vision display device, there are5760 (1920×RGB) source signal lines. When a source driver is not formedover the same substrate as the source signal lines, terminals of thesource signal lines are each connected to a terminal of the sourcedriver. Therefore, there is a problem in that a contact failure of aterminal easily occurs due to mechanical impact or the like. Reductionin the number of terminals is effective for less contact failure.Accordingly, an object is to reduce the number of terminals by formingan analog switch array over the same substrate as the source signallines and selectively connecting each terminal of RGB to a source driverin a time division manner.

FIG. 4B illustrates an equivalent circuit of the analog switch 1505. Inan example illustrated in FIG. 4B, 1920 output terminals of the sourcedriver and 3 terminals controlling gates of the analog switch arraybring the number of terminals connected to the outside of the displaydevice to 1923 in total, which is approximately one third of the numberof such terminals in the case where the analog switch array is not usedfor driving a source line. These terminals are connected to FPCs 1506,1507, 1508, and 1509. On the other hand, as compared to the case ofperforming time division, the source driver connected to the analogswitch needs to operate three times faster and time for writing sourcesignal lines needs to be reduced to one third. In order to reduce timefor writing source signal lines, it is important to improve currentcapability of transistors used for the analog switch.

FIG. 5A illustrates timing of performing time division. In the casewhere time division is not performed, a source line may be written inone line period. In the case where time division is performed, a sourceline needs to be written in one third or less of one line period.Moreover, in recent years, a double-frame-rate driving method, aquadruple-frame-rate driving method, and the like are widely used forimproving characteristics of moving images in display devices. Thesedriving methods aim to improve definitions of images by making imagesbetween frames in a television device, while television broadcasting isat normal speed.

Therefore, a display device is required to operate at a double framerate or a quadruple frame rate. FIG. 5B shows periods at a normal framerate, a double frame rate, and a quadruple frame rate. In FIG. 5A, apulse width “a” of a gate clock corresponds to one horizontal lineperiod, within which a source line needs to be written. As shown in FIG.5B, the value of “a” is 15.3 μs at a normal frame rate (a framefrequency of 60 Hz), 7.63 μs at a double frame rate (a frame frequencyof 120 Hz), and 3.81 μs at a quadruple frame rate (a frame frequency of240 Hz).

The display device needs to complete writing of a source line withinthese writing periods. By improvement of a mobility of a transistorincluding an oxide semiconductor, these can be satisfied. The “displaydevice” here means a display device including a transistor, such as aliquid crystal display device, an organic EL display device, or electricpaper.

Embodiment 2

FIGS. 6A and 6B each illustrate an embodiment of a display deviceprovided with a source driver. FIG. 6A illustrates an example in which apixel portion 1702, gate drivers 1703 and 1704, and a source driver 1705are provided over a glass substrate 1701. The gate drivers 1703 and 1704and the source driver 1705 are supplied with signals from FPCs 1706 and1707. A left side and a right side of the display device are providedwith the gate driver 1703 and the gate driver 1704, respectively, and apixel is driven; thus, driving can be performed with half of drivingcapability as compared to the case where a gate driver is provided ononly one side.

FIG. 6B illustrates an example in which, over a glass substrate 1711, apixel portion 1712, gate drivers 1713, 1714, 1715, and 1716 on an upperside and a lower side of a display device, and source drivers 1717,1718, 1719, and 1720 on a left side and a right side of the displaydevice are provided. The gate drivers and the source drivers aresupplied with signals from FPCs 1721, 1722, 1723, and 1724. With such anarrangement, display can be performed as long as each driver has drivingcapability for driving only one fourth of the display device. In thecase of a full high-definition display device, the display device can bedriven with drivers each having capability of driving a QHD (quarterhigh definition) display device. Therefore, according to the presentinvention, it is possible for a full high-definition display device witha size of approximately 100 inches to drive at a double frame rate witha transistor including an oxide semiconductor, whose field effectmobility is improved to higher than or equal to 50 cm²/Vs, preferably100 cm²/Vs.

Embodiment 3

Calculation results in the case where an analog switch is used fordriving a source line are show below. The calculation is made in thecase of a 100-inch liquid crystal display device with a frame frequencyof 240 Hz (quadruple frame rate). As described above, a source lineneeds to be written within 0.7 μs at a quadruple frame rate. At thistime, a transistor used for the analog switch for sampling has L/W=3μm/1500 μm, a field effect mobility of 100 cm²/Vs, and a thresholdvoltage of 1.5 V. A source signal line has a sheet resistance of 0.01Ω/square, a resistance of 2.08 KΩ, a capacity of 18.5 pF, and a linewidth of 6 μm. An aim is to perform writing until a source linepotential reaches 99.9% of an expected value.

FIG. 7 shows a relation between rise time of a gate line and a size of adisplay device. When the maximum delay time of the gate wiring is set to0.5 μs, the delay time of the gate line can satisfy the requirement evenwith a 100-inch display device. In this calculation, the sheetresistance of the gate line is 0.1 Ω/square, the capacity is 41.3 pF,and the line width is 23 μm.

FIG. 8 shows the calculation result of writing of a source line. Writingis performed while a sampling pulse is high, and operation is performedso that a potential of an input signal and a potential of the writtensource line become close. FIG. 8 shows a potential of the samplingpulse, a potential of the input signal, and a potential of the point ofthe source line whose potential has the biggest difference with theinput signal. As shown in FIG. 8, 99.9% of writing of the potential ofthe source line can be completed in 0.2 μs after rising of the inputsignal. Thus, a 100-inch full high-definition display device, whichincorporates an analog switch for driving a source line, can be drivenat quadruple frame rate by improving a mobility of a transistorincluding an oxide semiconductor. Although calculation is made with a100-inch display device here, the size of the display device is notlimited to this. A display device with a size of 100 inches or smalleris acceptable. Moreover, a display device with a size of 100 inches orlarger is acceptable with further improvement in a field effect mobilityand reduction in wiring resistance.

Embodiment 4

A calculation result in the case where a shift register is used as asource driver for driving a source line is explained. A set/reset shiftregister illustrated in FIG. 9 is used. Further, the calculation is madeon the assumption that the source drivers are arranged as illustrated inFIG. 6B. The sampling is performed for 960 shift registers at a time.One quarter of a region of a full high-definition display device iswritten by one source driver, so that points to be sampled are960×RGB=2880. The number of stages of shift registers required forsampling the 960 shift registers at a time is 30 stages. FIG. 10A showsa timing chart of this case. FIG. 10B shows periods set in accordancewith FIG. 10A. The clock frequency of the source driver corresponds tothe inverse of the double of the time period B shown in FIG. 10B.

The clock frequency needed for driving the display device is as follows:579 kHz in the case of driving at a normal frame rate; 1.15 MHz in thecase of driving at a double frame rate; and 2.31 MHz in the case ofdriving at a quadruple frame rate. The shift registers can operate underthe above condition only when field effect mobility is 100 cm²/Vs. Inthis case, allowable time for writing a source line is as follows: 0.43μs at a normal frame rate; 0.22 μs at a double frame rate; and 0.11 μsat a quadruple frame rate. This is enough for a capability of a sourcedriver, but the delay time of the source driver is a problem. In thecase of a 100-inch display device, actual driving corresponds to thecase of a 50-inch display device; accordingly, conditions of the sourceline at this time are as follows: the sheet resistance of the sourcesignal line is 0.01 Ω/square; the resistance of the source line is 1.04KΩ; the capacitance of the source line is 9.3 pF; and the line width is20 μm.

FIG. 11 shows the delay time of the source line. In the case of a100-inch display device, the wiring delay is 30 ns; therefore, writingneeds to be completed within approximately 60% of the allowable time.Accordingly, it is difficult for the 100-inch display device to performwriting. FIG. 11 shows data of a 10-inch display device with the framefrequency at a quadruple frame rate. Here, a potential of a samplingpulse, a potential of an input signal, and a potential of a point of thesource line, whose potential has the biggest difference with the inputsignal are shown. Writing is performed in a period when the samplingpulse is high and operation is performed so that the potential of theinput signal is equal to the potential of the source line. The potentialof the input signal reaches 99.9% of its maximum in approximately 0.07μs after rising of the input signal, which indicates that a 10-inchdisplay device can operate at a quadruple frame rate.

FIG. 12 shows a calculation result in the case where the frame frequencyis 120 Hz at double speed and the size of a display device is 100inches. Conditions other than the frequency are the same as the above.Here, a potential of a sampling pulse, a potential of an input signal,and a potential of a point of the source line, whose potential has thebiggest difference with the input signal are shown. Writing is performedin a period when the sampling pulse is high and operation is performedso that the potential of the input signal is equal to the potential ofthe source line. The potential of the input signal reaches 99.9% of itsmaximum in approximately 0.13 μs after rising of the input signal. Inthis case, this indicates that 99.9% of writing can be performed withinallowable time even with a 100-inch display device. Thus, with amobility of 100 cm²/Vs, a display device even with a display size of 100inches can operate at a double frame rate with source driversincorporated.

Embodiment 5

In this embodiment, an example of process for manufacturing a transistorwill be described with reference to FIG. 1A to 1C, FIGS. 2A to 2E, andFIGS. 3A and 3B.

First, a conductive film is formed over a substrate 400 which is asubstrate having an insulating surface, and then a gate electrode layer401 is provided using a photolithography step with the use of aphotomask.

As the substrate 400, a glass substrate which enables mass production ispreferably used, in particular. As a glass substrate used for thesubstrate 400, a glass substrate whose strain point is higher than orequal to 730° C. may be used when the temperature of the heat treatmentto be performed in a later step is high. For the substrate 400, forexample, a glass material such as aluminosilicate glass,aluminoborosilicate glass, or barium borosilicate glass is used. Notethat by containing a larger amount of barium oxide (BaO) than boronoxide, a heat-resistant glass substrate which is of more practical usecan be formed. Therefore, a glass substrate which contains BaO and B₂O₃so that the amount of BaO is larger than that of B₂O₃ is preferablyused.

An insulating layer serving as a base layer may be provided between thesubstrate 400 and the gate electrode layer 401. The base layer has afunction of preventing diffusion of an impurity element from thesubstrate 400, and can be formed with a single-layer or a stacked-layerstructure using one or more of a silicon nitride layer, a silicon oxidelayer, a silicon nitride oxide layer, and a silicon oxynitride layer.

A metal conductive layer can be used as the gate electrode layer 401. Asthe material of the metal conductive layer, an element selected from Al,Cr, Cu, Ta, Ti, Mo, and W, an alloy containing any of these elements asa component, an alloy containing any of these elements in combination,or the like is preferably used. For example, a three-layer structure inwhich an aluminum layer is stacked over a titanium layer and a titaniumlayer is stacked over the aluminum layer, or a three-layer structure inwhich an aluminum layer is stacked over a molybdenum layer and amolybdenum layer is stacked over the aluminum layer is preferable.Needless to say, the metal conductive layer may have a single-layerstructure, a two-layer structure, or a structure in which four or morelayers are stacked. In the case where heat treatment is performed later,a material which can withstand a temperature of the heat treatment ispreferably selected for the gate electrode layer 401.

Next, a gate insulating layer 402 is formed over the gate electrodelayer 401. The gate insulating layer 402 can be formed to have a singlelayer of a silicon oxide layer, a silicon nitride layer, a hafnium oxidelayer, a silicon oxynitride layer, or a silicon nitride oxide layer, ora stacked layer thereof by a plasma CVD method, a sputtering method, orthe like. For example, a stack including a silicon nitride film and asilicon oxide film is used. The thickness of the gate insulating layer402 is greater than or equal to 50 nm and less than or equal to 200 nm.

In this embodiment, the gate insulating layer 402 is formed using ahigh-density plasma apparatus. Here, a high-density plasma apparatusrefers to an apparatus which can realize a plasma density higher than orequal to 1×10¹¹/cm³. For example, plasma is generated by applying amicrowave power higher than or equal to 3 kW and lower than or equal to6 kW so that the insulating film is formed.

A monosilane gas (SiH₄), nitrous oxide (N₂O), and a rare gas areintroduced into a chamber as a source gas to generate high-densityplasma at a pressure higher than or equal to 10 Pa and lower than orequal to 30 Pa so that the insulating film is formed over a substratehaving an insulating surface, such as a glass substrate. After that, thesupply of a monosilane gas is stopped, and nitrous oxide (N₂O) and arare gas are introduced without exposure to the air, so that plasmatreatment may be performed on a surface of the insulating film. Theplasma treatment performed on the surface of the insulating film byintroducing nitrous oxide (N₂O) and a rare gas is performed at leastafter the insulating film is formed. The insulating film formed throughthe above process procedure has small thickness and corresponds to aninsulating film whose reliability can be ensured even though it has athickness less than 100 nm, for example.

In forming the gate insulating layer 402, the flow ratio of a monosilanegas (SiH₄) to nitrous oxide (N₂O) which are introduced into the chamberis in the range of 1:10 to 1:200. In addition, as a rare gas which isintroduced into the chamber, helium, argon, krypton, xenon, or the likecan be used. In particular, argon, which is inexpensive, is preferablyused.

In addition, since the insulating film formed using the high-densityplasma apparatus can have certain thickness, the insulating film hasexcellent step coverage. Further, as for the insulating film formedusing the high-density plasma apparatus, the thickness of a thin filmcan be controlled precisely.

The insulating film formed through the above process procedure isgreatly different from an insulating film formed using a conventionalparallel plate PCVD apparatus. The etching rate of the insulating filmformed through the above process procedure is lower than that of theinsulating film formed using the conventional parallel plate PCVDapparatus by 10% or more or 20% or more in the case where the etchingrates with the same etchant are compared to each other. Thus, it can besaid that the insulating film formed using the high-density plasmaapparatus is a dense film.

In this embodiment, a silicon oxynitride film (also referred to asSiO_(x)N_(y), where x>y>0) with a thickness of 100 nm formed using thehigh-density plasma apparatus is used as the gate insulating layer 402.

Then, a first oxide semiconductor layer with a thickness greater than orequal to 2 nm and less than or equal to 15 nm is formed over the gateinsulating layer 402. The first oxide semiconductor layer can be formedby a sputtering method in a rare gas (typically, argon) atmosphere, anoxygen atmosphere, or a mixed atmosphere including a rare gas(typically, argon) and oxygen.

In addition, it is preferable that moisture or the like which remains inthe sputtering apparatus be removed before, during, or after depositionof the oxide semiconductor film. In order to remove residual moisture inthe sputtering apparatus, an adsorption-type vacuum pump may be used.Examples of pumps that can be used comprise a cryopump, an ion pump, ora titanium sublimation pump. The evacuation unit can be a turbo pumpprovided with a cold trap. In the sputtering apparatus which isevacuated with the cryopump, a hydrogen atom, a compound containing ahydrogen atom, such as water (H₂O), and the like are removed, wherebythe concentration of an impurity in the oxide semiconductor film formedin the deposition chamber can be reduced.

In this embodiment, the first oxide semiconductor layer with a thicknessof 5 nm is formed in an oxygen atmosphere, an argon atmosphere, or amixed atmosphere of argon and oxygen in the following conditions: anoxide semiconductor target (an In—Ga—Zn—O-based oxide semiconductortarget (In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio], that is, In:Ga:Zn=1:1:1[atomic ratio])) is used, the distance between the substrate and thetarget is 170 mm, pressure is 0.4 Pa, and a direct current (DC) powersource is 0.5 kW. Alternatively, as the oxide semiconductor target, atarget having a composition ratio of In:Ga:Zn=1:1:0.5 [atomic ratio] orIn:Ga:Zn=1:1:2 [atomic ratio] can be used. In this embodiment, sincecrystallization is intentionally caused by performing heat treatment ina later step, it is preferable to use a target of an oxide semiconductorin which crystallization is easily caused.

In addition, an oxide semiconductor contained in the target for an oxidesemiconductor has a relative density of 80% or more, preferably 95% ormore, more preferably 99.9% or more. The impurity concentration in theoxide semiconductor film which is formed using the target having highrelative density can be reduced, and thus a transistor having excellentelectric characteristics or high reliability can be obtained.

Further, pre-heat treatment is preferably performed before the firstoxide semiconductor layer is deposited, in order to remove moisture orhydrogen which remains on an inner wall of a sputtering apparatus, on asurface of the target, or inside a target material. As the pre-heattreatment, a method in which the inside of the deposition chamber isheated to higher than or equal to 200° C. and lower than or equal to600° C. under reduced pressure, a method in which introduction andexhaust of nitrogen or an inert gas are repeated while the inside of thedeposition chamber is heated, and the like can be given.

Next, the first oxide semiconductor layer is subjected to first heattreatment, so that at least part thereof is crystallized. In the firstheat treatment, a temperature higher than or equal to 450° C. and lowerthan or equal to 850° C. is employed. In addition, heating time islonger than or equal to 1 minute and shorter than or equal to 24 hours.By the first heat treatment, a first oxide semiconductor layer 403 thatis a polycrystalline layer obtained by crystal growth proceeding from asurface is formed (see FIG. 2A). Further, the crystal layer formed atthe surface has a surface parallel to an a-b plane and a c-axis alignedperpendicularly to the surface of the crystal layer. In this embodiment,an example is described in which the whole first oxide semiconductorfilm is made to contain a crystal (also referred to as a co-growing (CG)crystal) by the first heat treatment.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen, oxygen, or a raregas such as helium, neon, or argon. In addition, it is preferable thatthe purity of nitrogen, oxygen, or a rare gas such as helium, neon, orargon which is introduced to the heat treatment apparatus be 6N or more,more preferably 7N or more. Further, the first heat treatment may beperformed in a dry air atmosphere with an H₂O concentration of lowerthan or equal to 20 ppm.

In this embodiment, heat treatment in a dry air atmosphere at 700° C.for one hour is performed as the first heat treatment.

In addition, when the temperature is increased in the first heattreatment, the inside of a furnace may be set to a nitrogen atmosphere,and when cooling is performed, the inside of the furnace may be switchedto an oxygen atmosphere. By performing dehydration or dehydrogenation ina nitrogen atmosphere and switching the atmosphere to an oxygenatmosphere, oxygen can be supplied into the first oxide semiconductorlayer, so that an i-type oxide semiconductor layer can be obtained.

Next, a second oxide semiconductor layer 404 whose thickness is largerthan at least that of the first oxide semiconductor layer 403 and lessthan or equal to 10 μm is formed over the first oxide semiconductorlayer 403 that is a plate-shaped polycrystal (see FIG. 2B). Note that athickness of the second oxide semiconductor layer 404 suitable for adevice to be manufactured may be determined appropriately bypractitioners. For example, in the case of manufacturing a bottom-gatetransistor, the total thickness of the first oxide semiconductor layer403 and the second oxide semiconductor layer 404 may be greater than orequal to 10 nm and less than or equal to 200 nm.

As the second oxide semiconductor layer 404, a four-component metaloxide film such as an In—Sn—Ga—Zn—O film, a three-component metal oxidefilm such as an In—Ga—Zn—O film, an In—Sn—Zn—O film, an In—Al—Zn—O film,a Sn—Ga—Zn—O film, an Al—Ga—Zn—O film, or a Sn—Al—Zn—O film, atwo-component metal oxide film such as an In—Zn—O film, a Sn—Zn—O film,an Al—Zn—O film, a Zn—Mg—O film, a Sn—Mg—O film, or an In—Mg—O film, oran In—O film, a Sn—O film, or a Zn—O film can be used.

It is preferable that the first oxide semiconductor layer 403 and thesecond oxide semiconductor layer 404 be formed using materials includingthe same components or have the same crystal structures and latticeconstants close to each other (mismatch less than or equal to 1%). Inthe case where the materials including the same components are used,crystal growth with the use of the polycrystalline layer of the firstoxide semiconductor layer 403 as a seed is easily performed during thesubsequent crystallization step. In addition, in the case where thematerials including the same components are used, an interface propertysuch as adhesion or electric characteristics is favorable.

Next, second heat treatment is performed, so that crystal growth isperformed using the crystal layer of the first oxide semiconductor layer403 as a seed. The second heat treatment is performed at a temperaturehigher than or equal to 450° C. and lower than or equal to 850° C.,preferably higher than or equal to 550° C. and lower than or equal to650° C. In addition, heating time is longer than or equal to 1 minuteand shorter than or equal to 24 hours. By the second heat treatment, thesecond oxide semiconductor layer is crystallized. In this manner, aplate-shaped crystallized oxide semiconductor stack 430 can be obtained(see FIG. 2C).

A plate-shaped crystal is preferably a single crystal which isc-axis-aligned in a direction perpendicular to a surface of the oxidesemiconductor layer. If the plate-shaped crystal is not a single crystalbody, then the plate-shaped crystal is preferably a polycrystal in whichthe a-axis and the b-axis of each crystal are aligned in a channelformation region and is c-axis-aligned in a direction perpendicular to asurface of the oxide semiconductor layer. Note that in the case where asurface of the base of the oxide semiconductor layer is uneven, theplate-shaped crystal is a polycrystal.

In order to describe a superordinate concept to easily understand stepsof FIGS. 2A, 2B, and 2C, magnified schematic views are illustrated inFIGS. 1A, 1B, and 1C.

FIG. 1A illustrates a first oxide crystal component 501 which has beensubjected to first heat treatment for crystallization over a basecomponent 500. FIG 1A corresponds to FIG. 2A and the base component 500corresponds to the gate insulating layer 402. FIG. 1B corresponds toFIG. 2B and is a cross-sectional view at the time just after depositionof a second oxide semiconductor layer 502. FIG. 1C corresponds to FIG.2C and is a cross-sectional view at the time after second heattreatment. By the second heat treatment, a third oxide crystal component503 a formed of a crystal layer whose crystal direction is highlyaligned is obtained. In the case where the first oxide component and thesecond oxide component are formed using the oxide semiconductormaterials including the same components, as illustrated in FIG. 1C,crystal growth proceeds upward toward a surface of the second oxidecomponent with the use of a crystal layer of the third oxide crystalcomponent 503 a as a nucleus, and a second oxide crystal component 503 bis formed, so that all the crystal components are aligned along theirc-axis. Therefore, a boundary between the third oxide crystal componentand the second oxide crystal component is unclear as illustrated with adotted line in FIG. 1C. In addition, by the second heat treatment, theinside of the second oxide component which has just been deposited ishighly purified to be layer including an amorphous oxide semiconductor.

Next, the oxide semiconductor stack 430 formed from the first oxidesemiconductor layer and the second oxide semiconductor layer isprocessed into an island-shaped oxide semiconductor stack 431 using aphotolithography step (see FIG. 2D). Further, a resist mask for formingthe island-shaped oxide semiconductor stack 431 may be formed using aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

Next, after a metal conductive film is formed over the gate insulatinglayer 402 and the island-shaped oxide semiconductor stack 431 by asputtering method or the like, photolithography step is performed sothat a resist mask is formed. Then, the metal conductive film isselectively etched, so that metal electrode layers are formed.

As the material of the metal conductive film, which is to be a sourceelectrode and a drain electrode (including a wiring formed using thesame layer), a metal material such as Al, Cu, Cr, Ta, Ti, Mo, or W; oran alloy material whose main component is any of these metal materialsis used. In addition, a structure in which a layer of ahigh-melting-point metal such as Cr, Ta, Ti, Mo, or W is stacked on alower side and/or an upper side of a metal layer of Al, Cu, or the likemay be used. In addition, heat resistance can be improved by using an Almaterial to which an element such as Si, Ti, Ta, W, Mo, Cr, Nd, Sc, or Ywhich prevents generation of a hillock or a whisker in an Al film isadded.

For example, the metal conductive film preferably has a three-layerstructure in which an aluminum layer is stacked over a titanium layerand a titanium layer is stacked over the aluminum layer, or a threelayer structure in which an aluminum layer is stacked over a molybdenumlayer and a molybdenum layer is stacked over the aluminum layer.Alternatively, the metal conductive film can have a two-layer structurein which an aluminum layer and a tungsten layer are stacked, a two-layerstructure in which a copper layer and a tungsten layer are stacked, or atwo-layer structure in which an aluminum layer and a molybdenum layerare stacked. Needless to say, the metal conductive film may have asingle-layer structure or a stacked-layer structure including four ormore layers.

Then, the resist mask is removed, and a photolithography step isperformed. A resist mask is formed and selective etching is performed,so that a source electrode layer 405 a and a drain electrode layer 405 bare formed. After that, the resist mask is removed (see FIG. 2E). Notethat in this photolithography step, in some cases, part of theisland-shaped oxide semiconductor stack 431 is etched, whereby an oxidesemiconductor layer having a groove (a depression portion) is formed.

As illustrated in FIG. 2E, the gate electrode layer 401 includes aregion overlapping with the source electrode layer 405 a (and the drainelectrode layer 405 b), which is also one feature. A region between anedge portion of the source electrode layer 405 a and a step of the gateinsulating layer 402, in other words, a region between the edge portionof the source electrode layer 405 a and a point at which a flat surfaceis changed to a tapered surface of the gate insulating layer in thecross-sectional view (here, an L_(OV) region in FIG. 2E) is included.The L_(OV) region is important in order that carriers are prevented fromflowing to a crystal grain boundary generated due to the step of theedge portion of the gate electrode layer.

In addition, in a side surface of the oxide semiconductor stack 432, acrystal layer which is in contact with the source electrode layer 405 aor the drain electrode layer 405 b is in an amorphous state in somecases.

Further, a resist mask used for forming the source electrode layer 405 aand the drain electrode layer 405 b may be formed by an inkjet method.Formation of the resist mask by an inkjet method needs no photomask;thus, manufacturing cost can be reduced.

In order to reduce the number of photomasks used in a photolithographystep and reduce the number of photolithography steps, an etching stepmay be performed with the use of a multi-tone mask which is alight-exposure mask through which light is transmitted to have aplurality of intensities. A resist mask formed with the use of amulti-tone mask has a plurality of thicknesses and further can bechanged in shape by etching; therefore, the resist mask can be used in aplurality of etching steps for processing into different patterns.Therefore, a resist mask corresponding to at least two kinds or more ofdifferent patterns can be formed by one multi-tone mask. Thus, thenumber of light-exposure masks can be reduced and the number ofcorresponding photolithography steps can be also reduced, wherebysimplification of a process can be realized.

Next, an oxide insulating layer 407 serving as a protective insulatingfilm is formed in contact with part of the oxide semiconductor layer.

The oxide insulating layer 407 can be formed to a thickness of at least1 nm by a method by which impurities such as water and hydrogen do notenter the oxide insulating layer 407, such as a sputtering method, asappropriate. In this embodiment, a 300-nm-thick silicon oxide film isdeposited as the oxide insulating layer 407 by a sputtering method. Thesubstrate temperature in deposition may be higher than or equal to roomtemperature and lower than or equal to 300° C. In this embodiment, thesubstrate temperature is 100° C. The deposition of the silicon oxidefilm by a sputtering method can be performed in a rare gas (typicallyargon) atmosphere, an oxygen atmosphere, or an atmosphere of a rare gas(typically argon) and oxygen. As a target, a silicon oxide target or asilicon target may be used. For example, with the use of a silicontarget, silicon oxide can be formed by a sputtering method in anatmosphere of oxygen and nitrogen. As the oxide insulating layer 407which is formed so as to be in contact with the oxide semiconductorlayer whose resistance is reduced, an inorganic insulating film is used.Typically, a silicon oxide film, a silicon nitride oxide film, analuminum oxide film, or an aluminum oxynitride film is used. Further, aprotective insulating layer such as a silicon nitride film or analuminum nitride film may be formed over the oxide insulating layer 407.

Further, a contact hole reaching the gate electrode layer 401 is formedin the oxide insulating layer 407 and the gate insulating layer 402, anda connection electrode which is electrically connected to the gateelectrode layer 401 and supplies a gate potential to the gate electrodelayer 401 may be formed over the oxide insulating layer 407.Alternatively, the following may be employed: a contact hole reachingthe gate electrode layer 401 is formed after formation of the gateinsulating layer 402; a connection electrode is formed thereover usingthe same material as that of the source electrode layer and the drainelectrode layer; the oxide insulating layer 407 is formed over theconnection electrode; a contact hole reaching the connection electrodeis formed in the oxide insulating layer 407; and then, an electrodewhich is electrically connected to the connection electrode and suppliesa gate potential to the connection electrode is formed over the oxideinsulating layer 407.

Through the above process, a transistor 470 is formed (see FIG. 3B).FIG. 3A illustrates an example of a top view of the transistor 470. Notethat FIG. 3B corresponds to a cross-sectional view taken along a brokenline C1-C2 in FIG. 3A.

One of features of the transistor 470 is that a top surface of the gateelectrode layer which overlaps with the channel formation region isflat, an oxide component which is c-axis-aligned perpendicularly to theflat surface is included, and the source electrode layer or the drainelectrode layer overlaps with the step generated due to the edge portionof the gate electrode layer. In the case where there is a projectionportion and a recessed portion on the substrate side, in the oxidecomponent (the oxide semiconductor stack 432 in this embodiment), aregion in which crystal growth meets the recessed portion becomespolycrystals having a crystal boundary. Accordingly, by forming theL_(OV) region illustrated in FIG. 3B, carriers can be prevented fromflowing to the crystal boundary generated in unevenness of the edgeportion of the gate electrode layer. Therefore, in the transistor 470,the source electrode layer or the drain electrode layer is provided overpart of the flat portion of the gate electrode layer, so that the sourceelectrode layer or the drain electrode layer includes a regionoverlapping with the gate electrode layer.

Needless to say, this embodiment is not particularly limited to thestructure of the transistor 470 illustrated in FIG. 3B. A top gatetransistor or a bottom gate transistor may be employed. For example, inorder to prevent etching damage at the time of formation of the sourceelectrode layer and the drain electrode layer in FIG. 2E, a channel-stopstructure in which an oxide insulating layer overlapping with a channelformation region is formed as a channel stopper may be employed for thetransistor.

In addition, an electrode layer which can function as a back gate may beprovided over the oxide insulating layer 407. The potential of the backgate can be a fixed potential, e.g., 0 V, or a ground potential, and maybe determined as appropriate by practitioners. Further, by controllinggate voltage applied to the back gate, threshold voltage can becontrolled. Alternatively, when the threshold voltage is set positive,the transistor can function as an enhancement type transistor. Furtheralternatively, when the threshold voltage is set negative, thetransistor can function as a depletion type transistor. For example, aninverter circuit including a combination of the enhancement typetransistor and the depletion type transistor (hereinafter, such acircuit is referred to as an EDMOS circuit) can be used for a drivercircuit. The driver circuit includes at least a logic circuit portion,and a switch portion or a buffer portion. The logic circuit portion hasa circuit structure including the above EDMOS circuit.

A principle of operation of a bottom-gate transistor including an oxidesemiconductor will be described below.

FIG. 13 is a longitudinal cross-sectional view of a transistor includingan oxide semiconductor. An oxide semiconductor layer (OS) is providedover a gate electrode (GE) with a gate insulating film (GI) providedtherebetween, and a source electrode (S) and a drain electrode (D) areprovided thereover. In addition, an oxide insulating layer overlappingwith a channel formation region of the oxide semiconductor layer (OS) isprovided the source electrode (S) and the drain electrode (D).

FIGS. 14A and 14B are energy band diagrams (schematic diagrams) alongthe cross section A-A′ illustrated in FIG. 13. FIG. 14A shows the casewhere the source and the drain have voltage of the same potential(V_(D)=0 V). FIG. 14B shows the case where positive potential is appliedto the drain (V_(D)>0) with respect to the source. In FIG. 14B, a blackcircle () and a white circle (◯) represent an electron and a hole andhave electric charges (−q, +q), respectively.

FIGS. 15A and 15B are energy band diagrams (schematic views) taken alongthe cross section B-B′ of FIG. 13 in the case where a gate voltage is 0V. FIG. 15A shows an on state in which a positive potential (+V_(G)) isapplied to the gate (GE) and carriers (electrons) flow between thesource and the drain. FIG. 15B shows a state in which a negativepotential (−V_(G)) is applied to the gate (GE), that is, a case wherethe transistor is in an off state (where minority carriers do not flow).

When the thickness of the oxide semiconductor is approximately 50 nm anda donor concentration becomes less than or equal to 1×10¹⁸/cm³ by highlypurifying the oxide semiconductor, a depletion layer spreads throughoutthe oxide semiconductor. In other words, the transistor can be regardedas a fully-depletion type transistor.

FIG. 16 shows relation between the vacuum level and the work function ofa metal (φM) and relation between the vacuum level and the electronaffinity (χ) of an oxide semiconductor.

Because metal degenerates, the conduction band and the Fermi levelcorrespond to each other. On the other hand, a conventional oxidesemiconductor is typically an n-type semiconductor; in that case, theFermi level (Ef) is away from the intrinsic Fermi level (Ei) located inthe middle of a band gap and is located closer to the conduction band.Note that it is known that hydrogen is a donor in an oxide semiconductorand is one factor causing an oxide semiconductor to be an n-typesemiconductor.

On the other hand, an oxide semiconductor of the present invention is anintrinsic (i-type) or substantially intrinsic oxide semiconductor whichis obtained by removing hydrogen that is an n-type impurity from anoxide semiconductor and purifying the oxide semiconductor so that animpurity that is not a main component of the oxide semiconductor iscontained therein as little as possible. In other words, a feature isthat a purified i-type (intrinsic) semiconductor, or a semiconductorclose thereto, is obtained not by adding an impurity but by removing animpurity such as hydrogen or water as much as possible. This enables theFermi level (Ef) to be at the same level as the intrinsic Fermi level(Ei).

An oxide semiconductor has a band gap (Eg) of 3.05 eV to 3.15 eV. In thecase where the band gap (Eg) of an oxide semiconductor is 3.15 eV, theelectron affinity (χ) is said to be 4.3 eV. The work function oftitanium (Ti) used for forming the source and drain electrodes issubstantially equal to the electron affinity (χ) of the oxidesemiconductor. In that case, a Schottky barrier for electrons is notformed at an interface between the metal and the oxide semiconductor.

In other words, in the case where the work function of the metal (φM)and the electron affinity (χ) of the oxide semiconductor are equal toeach other and the metal and the oxide semiconductor are in contact witheach other, an energy band diagram (a schematic diagram) as illustratedin FIG. 14A is obtained.

In FIG. 14B, a black circle () represents an electron, and when apositive potential is applied to the drain, the electron is injectedinto the oxide semiconductor over the barrier (h) and flows toward thedrain. In that case, the height of the barrier (h) changes depending onthe gate voltage and the drain voltage; in the case where a positivedrain voltage is applied, the height of the barrier (h) is smaller thanthe height of the bather in FIG. 14A where no voltage is applied, thatis, ½ of the band gap (Eg).

At this time, the electron moves in the bottom, which is energeticallystable, on the oxide semiconductor side at the interface between thegate insulating film and the highly-purified oxide semiconductor asillustrated in FIG. 15A.

In addition, in FIG. 15B, when a negative potential (reverse bias) isapplied to the gate electrode (G1), the value of current is extremelyclose to zero because holes that are minority carriers are substantiallyzero.

By increasing the purity of the oxide semiconductor so that the oxidesemiconductor includes an impurity other than its main component aslittle as possible, an intrinsic (i-type) or substantially intrinsicoxide semiconductor is obtained. Consequently, the interfacecharacteristics with the gate insulating film become obvious. Therefore,the interface characteristics need to be considered separately from bulkcharacteristics. Thus, the gate insulating film is needed to form afavorable interface with the oxide semiconductor. For example, it ispreferable to use an insulating film which is formed by a CVD methodwhich uses high density plasma generated with a power frequency from theVHF band to a microwave band or an insulating film formed by asputtering method.

The oxide semiconductor is highly purified and the interface between theoxide semiconductor and the gate insulating film is made favorable,whereby transistor characteristics of off current of less than or equalto 10⁻¹³ A and a subthreshold swing (S value) of 0.1 V/dec. (thethickness of the gate insulating film: 100 nm) are highly expected evenwhen the element has a channel width W of 1×10⁴ μm and a channel lengthof 3 μm.

As described above, the oxide semiconductor is highly purified so thatthe oxide semiconductor includes an impurity other than its maincomponent as little as possible, whereby a transistor with high mobilitycan be formed and favorable operation of the transistor can be obtained.

Embodiment 6

In this embodiment, the case where a first oxide component and a secondoxide component are formed using oxide semiconductor materials includingdifferent components is described, whereas the case where the firstoxide component and the second oxide component are formed using oxidesemiconductor materials including the same components is described inEmbodiment 5.

In this embodiment, a first oxide semiconductor layer with a thicknessof 5 nm is formed using a metal oxide target which does not include Gabut includes In and Zn at 1:1 [atomic ratio]. In the case of abottom-gate transistor, because oxide of Ga is an insulator, higherfield effect mobility is obtained in the case where an In—Zn—O film isused as the first oxide semiconductor layer than in the case where anIn—Ga—Zn—O film is used for the first oxide semiconductor layer.

Next, first heat treatment is performed. By the first heat treatment,crystal growth is performed from a surface of the first oxidesemiconductor layer, whereby a first oxide crystal component 531 whichincludes a polycrystal is formed so as to reach an interface between thefirst oxide semiconductor layer and a base component 520, although itdepends on conditions such as materials of the first oxide semiconductorlayer and the base component 520, heating temperature, and heating time(see FIG. 17A).

As the base component 520, an oxide layer, a metal layer, a nitridelayer, and the like can be given. By the first heat treatment, crystalgrowth of the first oxide crystal component 531 which includes apolycrystal whose crystal direction is relatively aligned proceeds fromthe surface of the first oxide semiconductor layer in the depthdirection regardless of the material of the base component. The firstoxide crystal component 531 is c-axis-aligned perpendicularly to thesurface.

FIG. 17B is a cross-sectional view at the time just after a second oxidecomponent 532 is deposited over the first oxide crystal component 531.In this embodiment, as the second oxide component 532, an In—Ga—Zn—Ofilm with a thickness of 50 nm is formed using a target for anIn—Ga—Zn—O-based oxide semiconductor (In:Ga:Zn=1:1:1[atomic ratio]).

Then, after deposition of the second oxide component 532, second heattreatment is performed. By the second heat treatment, crystal growth isperformed as illustrated in FIG. 17C. As illustrated in FIG. 17C,crystal growth proceeds upward toward a surface of the second oxidecomponent using a crystal layer of the first oxide crystal component 531as a seed, so that a second oxide crystal component 533 b is formed.

The first oxide crystal component 531 obtained by the first heattreatment is heated again by the second heat treatment to be a thirdoxide crystal component 533 a whose crystallinity is increased.

As the second oxide crystal component 532, an oxide semiconductormaterial whose component is different from that of the first oxidecrystal component 531 is used. Therefore, as illustrated in FIG. 17C, aboundary between the third oxide crystal component 533 a and the secondoxide crystal component 533 b is formed. In addition, also by the secondheat treatment, almost the whole first oxide semiconductor layerincluding the vicinity of an interface with a gate insulating layerincludes a polycrystal.

The structure of FIG. 17C can be referred to as a two-layer structure inwhich the first oxide crystal component 533 a is stacked on and incontact with the base component 520 and the second oxide crystalcomponent 533 b is stacked thereover. By using the different materials,the field effect mobility of the transistor can be increased. Inaddition, by using the In—Zn—O film which is crystallized more easilythan the In—Ga—Zn—O film as a seed of crystal growth, crystal growth isperformed upward with high efficiency, so that the In—Ga—Zn—O film caninclude a polycrystal

In addition, the case where the materials of the second oxide crystalcomponent in which crystal growth is performed and the first oxidecrystal component serving as a base are the same is referred to ashomoepitaxy. The case where the materials of the second oxide crystalcomponent in which crystal growth is performed and the first oxidecrystal component serving as a base are different is referred to asheteroepitaxy. In this embodiment, any of homoepitaxy and heteroepitaxycan be employed by selection of respective materials.

The conditions of the first heat treatment and the conditions of thesecond heat treatment are within a range of the conditions described inEmbodiment 5.

This embodiment can be freely combined with Embodiment 5.

Embodiment 7

In this embodiment, the case is described where a transistor including astacked oxide material having a crystal layer which is c-axis-aligned ismanufactured and a semiconductor device (also referred to as a displaydevice) having a display function, which include the transistors in apixel portion and further in a driver circuit is manufactured. Further,part or whole of a driver circuit using the transistor can be formedover the same substrate as a pixel portion, whereby a system-on-panelcan be obtained.

The display device includes a display element. As the display element, aliquid crystal element (also referred to as a liquid crystal displayelement) or a light-emitting element (also referred to as alight-emitting display element) can be used. The light-emitting elementincludes, in its category, an element whose luminance is controlled by acurrent or a voltage, and specifically includes, in its category, aninorganic electroluminescent (EL) element, an organic EL element, andthe like. Furthermore, a display medium whose contrast is changed by anelectric effect, such as electronic ink, can be used.

Further, the display device includes a panel in which a display elementis sealed. The display device relates to an embodiment of an elementsubstrate before the display element is completed in a manufacturingprocess of the display device, and the element substrate is providedwith a means for supplying a current to the display element in each of aplurality of pixels. Specifically, the element substrate may be in astate in which only a pixel electrode of the display element is formed,a state in which a conductive layer to be a pixel electrode is formedbut is not etched yet to form the pixel electrode, or any other states.

A display device in this specification means an image display device ora display device.

In this embodiment, an example of a liquid crystal display device isdescribed as a semiconductor device which is an embodiment of thepresent invention. First, an appearance and a cross section of a liquidcrystal display panel, which is an embodiment of a semiconductor device,will be described with reference to FIGS. 18A and 18B. FIG. 18A is a topview of a panel in which transistors 4010 and 4011 which include asemiconductor layer of a stacked oxide material including ac-axis-aligned crystal layer, and a liquid crystal element 4013, whichare formed over a first substrate 4001, are sealed between the firstsubstrate 4001 and a second substrate 4006 with a sealant 4005. FIG. 18Bcorresponds to a cross-sectional view taken along line M-N of FIG. 18A.

The sealant 4005 is provided so as to surround a pixel portion 4002, asignal line driver circuit 4003, and a scan line driver circuit 4004which are provided over the first substrate 4001. The second substrate4006 is provided over the pixel portion 4002, the signal line drivercircuit 4003, and the scan line driver circuit 4004. Thus, the pixelportion 4002, the signal line driver circuit 4003, and the scan linedriver circuit 4004 are sealed together with a liquid crystal layer 4008by the first substrate 4001, the sealant 4005, and the second substrate4006.

Further, the pixel portion 4002, the signal line driver circuit 4003,and the scan line driver circuit 4004 provided over the first substrate4001 each include a plurality of transistors. FIG. 18B illustrates thetransistor 4010 included in the pixel portion 4002 and the transistor4011 included in the scan line driver circuit 4004. Insulating layers4020 and 4021 are provided over the transistors 4010 and 4011.

As the transistors 4010 and 4011, a transistor including a stacked oxidematerial including a c-axis-aligned crystal layer described inEmbodiment 5 can be used. In this embodiment, the transistors 4010 and4011 are n-channel transistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel formation region of an oxidesemiconductor layer in the transistor 4011 for the driver circuit. Theconductive layer 4040 is provided at the position overlapping with thechannel formation region of the oxide semiconductor layer, whereby theamount of change in threshold voltage of the transistor 4011 betweenbefore and after the BT test can be reduced. The conductive layer 4040may have the same potential as or have potential different from that ofthe gate electrode layer of the transistor 4011 and can function as asecond gat electrode layer. Alternatively, the potential of theconductive layer 4040 may be GND or 0 V, or the conductive layer 4040may be in a floating state.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to the transistor 4010. A counter electrodelayer 4031 of the liquid crystal element 4013 is provided for the secondsubstrate 4006. A portion where the pixel electrode layer 4030, thecounter electrode layer 4031, and the liquid crystal layer 4008 overlapwith one another corresponds to the liquid crystal element 4013. Notethat the pixel electrode layer 4030 and the counter electrode layer 4031are provided with an insulating layer 4032 and an insulating layer 4033,respectively, which each function as an alignment film, and the liquidcrystal layer 4008 is sandwiched between the pixel electrode layer 4030and the counter electrode layer 4031 with the insulating layers 4032 and4033 therebetween.

Note that as the second substrate 4006, glass or plastics can be used.As plastic, a fiberglass-reinforced plastics (FRP) plate, a polyvinylfluoride (PVF) film, a polyester film, or an acrylic resin film can beused. In addition, a sheet with a structure in which an aluminum foil issandwiched between PVF films or polyester films can be used.

A columnar spacer 4035 which can be obtained in such a manner that aninsulating layer is selectively etched is provided to control a distance(a cell gap) between the pixel electrode layer 4030 and the counterelectrode layer 4031. Alternatively, a spherical spacer may also beused. The counter electrode layer 4031 is electrically connected to acommon potential line formed over the substrate where the transistor4010 is formed. In addition, with the use of a common connectionportion, the counter electrode layer 4031 and the common potential linecan be electrically connected to each other through conductive particlesarranged between the pair of substrates. Note that the conductiveparticles are included in the sealant 4005.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which appears just before a cholesteric phase changesinto an isotropic phase while temperature of cholesteric liquid crystalis increased. Since the blue phase appears within an only narrow rangeof temperature, liquid crystal composition containing a chiral agent at5 wt % or more so as to improve the temperature range is used for theliquid crystal layer 4008. The liquid crystal composition which includesa liquid crystal exhibiting a blue phase and a chiral agent has a shortresponse time of 1 msec or less, has optical isotropy, which makes thealignment process unneeded, and has a small viewing angle dependence.

When liquid crystal exhibiting a blue phase is used, rubbing treatmenton an alignment film is unnecessary; accordingly, electrostaticdischarge damage caused by the rubbing treatment can be prevented anddefects and damage of the liquid crystal display device in themanufacturing process can be reduced. Thus, productivity of the liquidcrystal display device can be increased. A transistor including an oxidesemiconductor layer particularly has a possibility that electricalcharacteristics of the transistor might significantly change and deviatefrom the designed range due to the influence of static electricity.Therefore, it is more effective to use a liquid crystal materialexhibiting a blue phase for a liquid crystal display device including atransistor including an oxide semiconductor layer.

Note that the liquid crystal display device described in this embodimentis an example of a transmissive liquid crystal display device; however,the liquid crystal display device can be applied to either a reflectiveliquid crystal display device or a semi-transmissive liquid crystaldisplay device.

An example of the liquid crystal display device described in thisembodiment is illustrated in which a polarizing plate is provided on theouter surface of the substrate (on the viewer side) and a coloring layerand an electrode layer used for a display element are provided on theinner surface of the substrate in that order; however, the polarizingplate may be provided on the inner surface of the substrate. The stackedstructure of the polarizing plate and the coloring layer is not limitedto this embodiment and may be set as appropriate depending on materialsof the polarizing plate and the coloring layer or conditions ofmanufacturing process. A light-blocking layer which functions as a blackmatrix may be provided when needed.

In this embodiment, in order to reduce surface unevenness of thetransistors and to improve reliability of the transistors, thetransistors are covered with the insulating layers (the insulating layer4020 and the insulating layer 4021) which function as protective layersor planarizing insulating layers. Note that the protective layer isprovided to prevent entry of a contaminant impurity such as an organicsubstance, a metal substance, or moisture floating in air, and ispreferably a dense film. The protective layer may be formed with asingle layer or a stacked layer of a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, an aluminum nitride layer, an aluminumoxynitride layer, and/or an aluminum nitride oxide layer by a sputteringmethod. In this embodiment, an example in which the protective layer isformed by a sputtering method is described; however, there is noparticular limitation on a method, and various kinds of methods may beused.

Here, the insulating layer 4020 having a stacked structure is formed asthe protective layer. As a first layer of the insulating layer 4020, asilicon oxide layer is formed by a sputtering method. The use of asilicon oxide layer as the protective layer provides an advantageouseffect of preventing hillock of an aluminum layer used for a sourceelectrode layer and a drain electrode layer.

An insulating layer is formed as a second layer of the protective layer.Here, as a second layer of the insulating layer 4020, a silicon nitridelayer is formed by a sputtering method. The use of the silicon nitridelayer as the protective layer can prevent ions such as sodium ions fromentering a semiconductor region, thereby suppressing variations inelectric characteristics of the transistors.

The insulating layer 4021 is formed as the planarizing insulating layer.As the insulating layer 4021, an organic material having heat resistancesuch as polyimide, acrylic, benzocyclobutene, polyamide, or epoxy can beused. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material), a siloxane-basedresin, PSG (phosphosilicate glass), BPSG (borophosphosilicate glass), orthe like. Note that the insulating layer 4021 may be formed by stackinga plurality of insulating layers formed of these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

A formation method of the insulating layer 4021 is not particularlylimited, and the following method can be employed depending on thematerial: a sputtering method, an SOG method, a spin coating method, adipping method, a spray coating method, a droplet discharge method(e.g., an ink-jet method, screen printing, or offset printing), or thelike. Further, the insulating layer 4021 can be formed with a doctorknife, a roll coater, a curtain coater, a knife coater, or the like. Inthe case of forming the insulating layer 4021 using a material solution,annealing (at 300° C. to 400° C.) of the semiconductor layer may beperformed at the same time as a baking step. The baking step of theinsulating layer 4021 also serves as annealing of the semiconductorlayer, whereby a semiconductor device can be manufactured efficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, indium tin oxide to which silicon oxide isadded, or the like.

Conductive compositions including a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the pixel electrodelayer 4030 and the counter electrode layer 4031. The pixel electrodeformed using the conductive composition preferably has a sheetresistance of less than or equal to 10000 ohms per square and atransmittance of greater than or equal to 70% at a wavelength of 550 nmFurther, the resistivity of the conductive high molecule included in theconductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive high molecule, a so-called π-electron conjugatedconductive polymer can be used. For example, polyaniline or a derivativethereof, polypyrrole or a derivative thereof, polythiophene or aderivative thereof, a copolymer of two or more kinds of them, and thelike can be given.

A variety of signals and potentials are supplied to the signal linedriver circuit 4003 which is formed separately, and the scan line drivercircuit 4004 or the pixel portion 4002 from an FPC 4018.

In this embodiment, a connection terminal electrode 4015 is formed usingthe same conductive layer as the pixel electrode layer 4030 included inthe liquid crystal element 4013. A terminal electrode 4016 is formedusing the same conductive layer as the source and drain electrode layersincluded in the transistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductivelayer 4019.

Note that FIGS. 18A and 18B illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, this embodiment is not limited to thisstructure. The scan line driver circuit may be formed over the samesubstrate as the pixel portion 4002, or only part of the signal linedriver circuit or part of the scan line driver circuit may be formedover the same substrate as the pixel portion 4002.

In addition, if needed, a color filter is provided for each of thepixels. In addition, a polarizing plate or a diffusion plate is providedon the outer sides of the first substrate 4001 and the second substrate4006. Further, a light source of a backlight is formed using acold-cathode tube or an LED. Thus, a liquid crystal display module isobtained.

For the liquid crystal display module, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, amulti-domain vertical alignment (MVA) mode, a patterned verticalalignment (PVA) mode, an axially symmetric aligned micro-cell (ASM)mode, an optically compensated birefringence (OCB) mode, a ferroelectricliquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC)mode, or the like can be used.

Through the above process, a highly reliable liquid crystal displaydevice can be manufactured.

By manufacturing a transistor in the driver circuit of the liquidcrystal display device using the method for manufacturing a stackedoxide material including a c-axis-aligned crystal layer which isdescribed in Embodiment 5, a normally-off transistor can be provided inthe driver circuit portion and power consumption can be reduced.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 8

An appearance and a cross section of a light-emitting display panel(also referred to as a light-emitting panel) which corresponds to oneembodiment of a semiconductor device will be described with reference toFIGS. 19A and 19B. FIG. 19A is a plan view of a panel in which atransistor including a stacked oxide material including a c-axis-alignedcrystal layer and a light-emitting element, which are formed over afirst substrate, are sealed between the first substrate and a secondsubstrate with a sealant. FIG. 19B is a cross-sectional view along lineH-I of FIG. 19A.

A sealant 4505 is provided so as to surround a pixel portion 4502,signal line driver circuits 4503 a and 4503 b, and scan line drivercircuits 4504 a and 4504 b which are provided over a first substrate4501. In addition, a second substrate 4506 is provided over the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b. Accordingly, the pixelportion 4502, the signal line driver circuits 4503 a and 4503 b, and thescan line driver circuits 4504 a and 4504 b are sealed together with afiller 4507 by the first substrate 4501, the sealant 4505, and thesecond substrate 4506. It is preferable that a panel be thus packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so that the pixel portion 4502,the signal line driver circuits 4503 a and 4503 b, and the scan linedriver circuits 4504 a and 4504 b are not exposed to the outside air.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of transistors, and atransistor 4510 included in the pixel portion 4502 and a transistor 4509included in the signal line driver circuit 4503 a are illustrated as anexample in FIG. 19B.

For the transistors 4509 and 4510, a highly reliable transistorincluding a stacked oxide material including a c-axis-aligned crystallayer described in Embodiment 5 can be employed. In this embodiment, thetransistors 4509 and 4510 are n-channel transistors.

A conductive layer 4540 is provided over part of the insulating layer4544, which overlaps with a channel formation region of an oxidesemiconductor layer in the transistor 4509 for the driver circuit. Theconductive layer 4540 is provided at the position overlapping with thechannel formation region of the oxide semiconductor layer, whereby theamount of change in threshold voltage of the transistor 4509 betweenbefore and after the BT test can be reduced. The conductive layer 4540may have the same potential as or have potential different from that ofthe gate electrode layer of the transistor 4509 and can function as asecond gate electrode layer. Alternatively, the potential of theconductive layer 4540 may be GND or 0 V, or the conductive layer 4540may be in a floating state.

In the transistor 4509, an insulating layer 4541 is formed in contactwith the semiconductor layer including the channel formation region, asa protective insulating layer. The insulating layer 4541 may be formedusing a material and a method which are similar to those of the oxideinsulating layer 407 described in Embodiment 5. Moreover, the insulatinglayer 4544 functioning as a planarizing insulating layer covers thetransistors in order to reduce surface unevenness of the transistors.Here, a silicon oxide layer is formed as the insulating layer 4541 by asputtering method in a manner similar to that of the oxide insulatinglayer 407 described in Embodiment 5.

The insulating layer 4544 is formed as the planarizing insulating layerover the insulating layer 4541. The insulating layer 4544 may be formedusing a material and a method which are similar to those of theinsulating layer 4021 described in Embodiment 7. Here, acrylic is usedfor the insulating layer 4544.

Moreover, reference numeral 4511 denotes a light-emitting element. Afirst electrode layer 4517 which is a pixel electrode included in thelight-emitting element 4511 is electrically connected to a source ordrain electrode layer of the transistor 4510. Note that the structure ofthe light-emitting element 4511 is, but not limited to, the stackstructure which includes the first electrode layer 4517, anelectroluminescent layer 4512, and the second electrode layer 4513. Thestructure of the light-emitting element 4511 can be changed asappropriate depending on the direction in which light is extracted fromthe light-emitting element 4511, or the like.

A partition wall 4520 is made of an organic resin layer, an inorganicinsulating layer, or organic polysiloxane. It is particularly preferablethat the partition wall 4520 be formed using a photosensitive materialand an opening be formed over the first electrode layer 4517 so that asidewall of the opening is formed as an inclined surface with continuouscurvature.

The electroluminescent layer 4512 may be formed with a single layer or aplurality of layers stacked.

A protective layer may be formed over the second electrode layer 4513and the partition wall 4520 in order to prevent oxygen, hydrogen,moisture, carbon dioxide, or the like from entering into thelight-emitting element 4511. As the protective layer, a silicon nitridelayer, a silicon nitride oxide layer, a DLC layer, or the like can beformed.

In addition, a variety of signals and potentials are supplied to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, or the pixel portion 4502 from FPCs 4518 aand 4518 b.

A connection terminal electrode 4515 is formed from the same conductivelayer as the first electrode layer 4517 included in the light-emittingelement 4511, and a terminal electrode 4516 is formed from the sameconductive layer as the source and drain electrode layers included inthe thin film transistors 4509 and 4510.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivelayer 4519.

As the second substrate located in the direction in which light isextracted from the light-emitting element 4511 needs to have alight-transmitting property. In that case, a light-transmitting materialsuch as a glass plate, a plastic plate, a polyester film, or an acrylicfilm is used.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (ethylenevinyl acetate) can be used. For example, nitrogen is used for thefiller.

In addition, if needed, an optical film, such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter, may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Through the above steps, a highly reliable light-emitting display device(display panel) can be manufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 9

An example of electronic paper will be described as an embodiment of asemiconductor device.

A transistor including a stacked oxide material including ac-axis-aligned crystal layer obtained by the method described inEmbodiment 5 may be used for electronic paper in which electronic ink isdriven by an element electrically connected to a switching element. Theelectronic paper is also referred to as an electrophoretic displaydevice (an electrophoretic display) and is advantageous in that it hasthe same level of readability as plain paper, it has lower powerconsumption than other display devices, and it can be made thin andlightweight.

Electrophoretic displays can have various modes. Electrophoreticdisplays contain a plurality of microcapsules dispersed in a solvent ora solute, each microcapsule containing first particles which arepositively charged and second particles which are negatively charged. Byapplying an electric field to the microcapsules, the particles in themicrocapsules move in opposite directions to each other and only thecolor of the particles gathering on one side is displayed. Note that thefirst particles and the second particles each contain pigment and do notmove without an electric field. Moreover, the first particles have acolor which is different from that of the second particles (theparticles may also be colorless).

Thus, an electrophoretic display is a display that utilizes a so-calleddielectrophoretic effect by which a substance having a high dielectricconstant moves to a high electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

When a plurality of the above microcapsules are arranged as appropriateover an active matrix substrate so as to be interposed between twoelectrodes, an active matrix display device can be completed, anddisplay can be performed by application of an electric field to themicrocapsules. For example, an active matrix substrate obtained using atransistor including a stacked oxide material including a c-axis-alignedcrystal layer described in Embodiment 5 can be used.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material of any ofthese.

FIG. 20 illustrates active matrix electronic paper as an example of asemiconductor device. A transistor 581 used for the semiconductor devicecan be manufactured in a manner similar to that of the transistordescribed in Embodiment 5 and is a highly reliable transistor includinga stacked oxide material including a c-axis-aligned crystal layer.

The electronic paper in FIG. 20 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare arranged between a first electrode layer and a second electrodelayer which are electrode layers used for a display element, and apotential difference is generated between the first electrode layer andthe second electrode layer to control orientation of the sphericalparticles, so that display is performed.

The transistor 581 is a bottom gate transistor and is covered with aninsulating layer 583 which is in contact with a semiconductor layer. Asource or drain electrode layer of the transistor 581 is in contact witha first electrode layer 587 through an opening formed in the insulatinglayer 583 and an insulating layer 585, whereby the thin film transistor581 is electrically connected to the first electrode layer 587. Betweena pair of substrates 580 and 596, spherical particles 589 each having ablack region 590 a, a white region 590 b, and a cavity 594 around theregions which is filled with liquid are provided between the firstelectrode layer 587 and a second electrode layer 588. A space around thespherical particles 589 is filled with a filler 595 such as a resin (seeFIG. 20).

The first electrode layer 587 corresponds to a pixel electrode, and thesecond electrode layer 588 corresponds to a common electrode. The secondelectrode layer 588 is electrically connected to a common potential lineprovided over the same insulating substrate as the transistor 581. Withthe use of a common connection portion, the second electrode layer 588can be electrically connected to the common potential line throughconductive particles provided between a pair of substrates.

Further, instead of the twisting ball, an electrophoretic element canalso be used. A microcapsule having a diameter of approximately 10 μm to200 μm in which transparent liquid, positively charged whitemicroparticles, and negatively charged black microparticles areencapsulated is used. In the microcapsule which is provided between thefirst electrode layer and the second electrode layer, when an electricfield is applied by the first electrode layer and the second electrodelayer, the white microparticles and the black microparticles move toopposite sides, so that white or black can be displayed. A displayelement using this principle is an electrophoretic display element andis generally called electronic paper. The electrophoretic displayelement has higher reflectance than a liquid crystal display element,and thus, an auxiliary light is unnecessary, power consumption is low,and a display portion can be recognized even in a dim place. Inaddition, even when power is not supplied to the display portion, animage which has been displayed once can be maintained. Accordingly, adisplayed image can be stored even if a semiconductor device having adisplay function (which may be referred to simply as a display device ora semiconductor device provided with a display device) is distanced froman electric wave source.

Through this process, highly reliable electronic paper can bemanufactured.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 10

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including game machines). Examplesof electronic appliances are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone (also referred to as a cellularphone or a mobile phone device), a portable game console, a portableinformation terminal, an audio reproducing device, a large-sized gamemachine such as a pachinko machine, and the like.

In this embodiment, examples of an electronic appliance mounted with adisplay device which can be obtained in any of Embodiments 7 to 9 aredescribed with reference to FIGS. 21A to 21E and FIG. 22.

FIG. 21A illustrates a laptop personal computer manufactured by mountingat least a display device as a component, which includes a main body3001, a housing 3002, a display portion 3003, a keyboard 3004, and thelike. Note that the laptop personal computer includes the liquid crystaldisplay device described in Embodiment 7.

FIG. 21B is a portable information terminal (PDA) manufactured bymounting at least a display device as a component, which includes adisplay portion 3023, an external interface 3025, an operation button3024, and the like in a main body 3021. A stylus 3022 is included as anaccessory for operation. Note that the portable information terminalincludes the light-emitting display device described in Embodiment 8.

FIG. 21C is an e-book reader manufactured by mounting the electronicpaper described in Embodiment 9 as a component. FIG. 21C illustrates ane-book reader 2700. For example, the e-book reader 2700 includes twohousings, a housing 2701 and a housing 2703. The housing 2701 and thehousing 2703 are combined with a hinge 2711 so that the e-book reader2700 can be opened and closed with the hinge 2711 as an axis. With sucha structure, the e-book reader 2700 can operate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 21C) and images can be displayed on a display portion onthe left side (the display portion 2707 in FIG. 21C).

FIG. 21C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, or the like may also beprovided on the surface of the housing, on which the display portion isprovided. Furthermore, an external connection terminal (an earphoneterminal, a USB terminal, a terminal that can be connected to an ACadapter and various cables such as a USB cable, or the like), arecording medium insertion portion, and the like may be provided on theback surface or the side surface of the housing. Moreover, the e-bookreader 2700 may have a function of an electronic dictionary.

The e-book reader 2700 may have a configuration capable of wirelesslytransmitting and receiving data. Through wireless communication, desiredbook data or the like can be purchased and downloaded from an electronicbook server.

FIG. 21D is a mobile phone manufactured by mounting at least a displaydevice as a component, which includes two housings: a housing 2800 and ahousing 2801. The housing 2801 is provided with a display panel 2802, aspeaker 2803, a microphone 2804, a pointing device 2806, a camera lens2807, an external connection terminal 2808, and the like. The housing2801 is also provided with a solar battery cell 2810 for charging of theportable information terminal, an external memory slot 2811, and thelike. Further, an antenna is incorporated in the housing 2801.

The display panel 2802 is provided with a touch panel. A plurality ofoperation keys 2805 which is displayed as images is illustrated bydashed lines in FIG. 21D. Note that a booster circuit by which a voltageoutput from the solar cell 2810 is increased to be sufficiently high foreach circuit is also included.

In the display panel 2802, the display direction can be changed asappropriate depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus can be used as a videophone. The speaker 2803 andthe microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousings 2800 and 2801 in a state where they are developed asillustrated in FIG. 21D can be slid so that one is lapped over theother; therefore, the size of the mobile phone can be reduced, whichmakes the mobile phone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored by inserting a storage medium into theexternal memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 21E is a digital camera manufactured by mounting at least a displaydevice as a component, which includes a main body 3051, a displayportion (A) 3057, an eyepiece 3053, operation switches 3054, a displayportion (B) 3055, a battery 3056, and the like.

FIG. 22 illustrates a television set 9600. In the television set 9600, adisplay portion 9603 is incorporated in a housing 9601. The displayportion 9603 can display images. Here, the housing 9601 is supported bya stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610, whereby an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set 9600 isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

In the display portion 9603, a plurality of transistors described inEmbodiment 5 is provided as switching elements of pixels. As a drivercircuit formed over the same insulating substrate as the display portion9603, a transistor with high mobility, which is described in Embodiment5, is provided.

This embodiment can be freely combined with any one of Embodiments 1 to9.

This application is based on Japanese Patent Application serial no.2009-276918 filed with Japan Patent Office on Dec. 4, 2009, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCES

400: substrate, 401: gate electrode layer, 402: gate insulating layer,403: first oxide semiconductor layer, 404: second oxide semiconductorlayer, 405 a: source electrode layer, 405 b: drain electrode layer, 407:oxide insulating layer, 430: oxide semiconductor stack, 431:island-shaped oxide semiconductor stack, 432: oxide semiconductor stack,470: transistor, 500: base component, 501: first oxide crystalcomponent, 502: oxide semiconductor layer, 503 a: oxide crystalcomponent, 503 b: oxide crystal component, 520: base component, 531:oxide crystal component, 532: oxide component, 533 a: oxide crystalcomponent, 533 b: oxide crystal component, 580: substrate, 581:transistor, 583: insulating layer, 585: insulating layer, 587: electrodelayer, 588: electrode layer, 589: spherical particle, 590 a: blackregion, 580 b: white region, 594: cavity, 595: filler, 596: substrate,1501: glass substrate, 1502: pixel portion, 1503: gate driver, 1504:gate driver, 1505: analog switch, 1506: FPC, 1507: FPC, 1508: FPC, 1509:FPC, 1701: glass substrate, 1702: pixel portion, 1703: gate driver,1704: gate driver, 1705: source driver, 1706: FPC, 1707: FPC, 1711:glass substrate, 1712: pixel portion, 1713: gate driver, 1714: gatedriver, 1715: gate driver, 1716: gate driver, 1717: source driver, 1718:source driver, 1719: source driver, 1720: source driver, 1721: FPC,1722: FPC, 1723: FPC, 1724: FPC, 2700: e-book reader, 2701: housing,2703: housing, 2705: display portion, 2707: display portion, 2711:hinge, 2721: power switch, 2723: operation key, 2725: speaker, 2800:housing, 2801: housing, 2802: display panel, 2803: speaker, 2804:microphone, 2805: operation key, 2806: pointing device, 2807: cameralens, 2808: external connection terminal, 2810: solar battery cell,2811: external memory slot, 3001: main body, 3002: housing, 3003:display portion, 3004: keyboard, 3021: main body, 3022: stylus, 3023:display portion, 3024: operation button, 3025: external interface, 3051:main body, 3053: eyepiece, 3054: operation switch, 3055: display portion(B), 3056: battery, 3057: display portion (A), 4001: substrate, 4002:pixel portion, 4003: signal line driver circuit, 4004: scan line drivercircuit, 4005: sealant, 4006: substrate, 4008: liquid crystal layer,4010: transistor, 4011: transistor, 4013: liquid crystal element, 4015:connection terminal electrode, 4016: terminal electrode, 4018: FPC,4019: anisotropic conductive layer, 4020: insulating layer, 4021:insulating layer, 4030: pixel electrode layer, 4031: counter electrodelayer, 4032: insulating layer, 4033: insulating layer, 4035: spacer,4040: conductive layer, 4501: substrate, 4502: pixel portion, 4503 a:signal line driver circuit, 4503 b: signal line driver circuit, 4504 a:scan line driver circuit, 4504 b: scan line driver circuit, 4505:sealant, 4506: substrate, 4507: filler, 4509: transistor, 4510:transistor, 4511: light-emitting element, 4512: electroluminescentlayer, 4513: electrode layer, 4515: connection terminal electrode, 4516:terminal electrode, 4517: electrode layer, 4518 a: FPC, 4518 b: FPC,4519: anisotropic conductive layer, 4520: partition wall, 4540:conductive layer, 4541: insulating layer, 4544: insulating layer, 9600:television set, 9601: housing, 9603: display portion, 9605: stand, 9607:display portion, 9609: operation key, and 9610: remote controller.

1. A display device including a plurality of pixels, a plurality ofsignal lines, and a plurality of scan lines over a substrate, andcomprising: transistors with a field effect mobility of 50 cm²/Vs orhigher over the substrate; and a gate driver and an analog switch eachincluding at least one of the transistors, the analog switch beingconfigured for driving a source line, wherein the transistors comprisean oxide semiconductor layer.
 2. The display device according to claim1, wherein the oxide semiconductor layer has crystallinity.
 3. Thedisplay device according to claim 1, wherein the oxide semiconductorlayer comprises a crystal having a c-axis aligned perpendicularly to asurface of the oxide semiconductor layer.
 4. The display deviceaccording to claim 1, wherein the field effect mobility of each of thetransistors is of 100 cm²/Vs or higher.
 5. The display device accordingto claim 1, wherein a diagonal of a display panel is longer than orequal to 20 inches.
 6. The display device according to claim 1, whereina gate electrode of each of the transistors comprises at least one ofaluminum, copper, chrome, tantalum, titanium molybdenum, and tungsten.7. A display device including a plurality of pixels, a plurality ofsignal lines, and a plurality of scan lines over a substrate, andcomprising: transistors with a field effect mobility of at least 50cm²/Vs or higher over the substrate; and a gate driver and a sourcedriver each including at least one of the transistors, wherein thetransistors comprise an oxide semiconductor layer.
 8. The display deviceaccording to claim 7, wherein the oxide semiconductor layer hascrystallinity.
 9. The display device according to claim 7, wherein theoxide semiconductor layer comprises a crystal having a c-axis alignedperpendicularly to a surface of the oxide semiconductor layer.
 10. Thedisplay device according to claim 7, wherein the field effect mobilityof each of the transistors is of 100 cm²/Vs or higher.
 11. The displaydevice according to claim 7, wherein a diagonal of a display panel islonger than or equal to 20 inches.
 12. The display device according toclaim 7, wherein a gate electrode of each of the transistors comprisesat least one of aluminum, copper, chrome, tantalum, titanium molybdenum,and tungsten.
 13. The display device according to claim 7, wherein thegate driver and the source driver are formed over the substrate.
 14. Adisplay device including a plurality of pixels, a plurality of signallines, and a plurality of scan lines over a substrate, and comprising: afirst transistor in one of the pixels, a gate driver including a secondtransistor, and a source driver including a third transistor, whereineach of the first to third transistors has a field effect mobility of 50cm²/Vs or higher, and wherein each of the first to third transistorscomprises an oxide semiconductor layer.
 15. The display device accordingto claim 14, wherein the oxide semiconductor layer has crystallinity.16. The display device according to claim 14, wherein the oxidesemiconductor layer comprises a crystal having a c-axis alignedperpendicularly to a surface of the oxide semiconductor layer.
 17. Thedisplay device according to claim 14, wherein the field effect mobilityof each of the first to third transistors is of 100 cm²/Vs or higher.18. The display device according to claim 14, wherein a diagonal of adisplay panel is longer than or equal to 20 inches.
 19. The displaydevice according to claim 14, wherein a gate electrode of each of thefirst to third transistors comprises at least one of aluminum, copper,chrome, tantalum, titanium molybdenum, and tungsten.